The present invention relates to a method and apparatus for removing sand and other debris from a wellbore and in particular, though not exclusively, to a downhole mixing tool which combines the sand with a treatment fluid so that the sand is lifted to the surface of the well for removal.
During production from an oil well, the oil fluid typically contains debris and foreign particles. Sand is a particular problem as it is swept out of the producing formation and the quantity found in the fluid can increase as oil production increases. Sufficient sand can build up in the wellbore to prevent efficient production.
Debris catchers have been developed which are run into the wellbore and filter sand and other debris from the fluid stream. The clean fluid is brought to the surface while the debris is held in a container. The main disadvantages with this approach are that the containers need to be emptied, requiring the tool to be pulled and run again; it is difficult to determine when a container is full so needless runs are made or no filtering is taking place; the filters can become blocked with debris; and the valves, which are typically used at an the entrance to the container, can also fail due to the build up of debris, thereby expelling the debris back into the wellbore as the tool is removed.
An alternative technique is to attempt to circulate the debris out of the wellbore. Fluid is pumped down a tubular string where upon it mixes with the sand and lifts it to the surface in the annulus between the string and the wall of the wellbore. This has been seen as inefficient as the circulating fluid is ineffectual at breaking up the sand and entraining it within the fluid. Additionally, due to the volume of debris suspended fluid which must be lifted in the annulus of a typical wellbore, insufficient annular velocity is available and the debris settles back out of the fluid.